Magnetic heads are used to read data from and write data onto certain data regions of magnetic disks. To accurately perform these operations, it is critical that the heads be properly positioned over the data regions. In an attempt to ensure the proper positioning of the heads relative to the data regions, disk drive manufacturers have resorted to using servo-systems. The servo-systems sense the position of the heads and generate position error ("PES") signals to adjust the position of the heads to their respective proper locations over the magnetic disk. The magnitude of the PES signal varies based upon the distance the heads are from their proper locations. Generally, the greater the magnitude of the PES signal the greater the distance the heads are from their proper locations.
In completely linear systems, a disk drive's servo-system determines the distance and direction a head is to be moved by multiplying a measured PES signal by a constant. In non-linear systems, however, a disk drive's servo-system determines which direction and how far to move a head based upon a PES curve which is stored within the servo-system's memory. The establishment of an accurate PES curve is, therefore, critical to the operation of non-linear systems.
In non-linear systems, a small change in the measured PES signal can cause the servo-system to instruct the head to move great distances. For this reason, small errors in the measured PES signal can cause large errors in the head position To minimize such large errors, it is desirable to develop PES curves which are relatively linear.
The present invention provides a method and apparatus of developing a relatively linear PES curve for a non-linear system.
Additionally, because PES curves in a non-linear system can vary from head to head based upon varying head widths during their manufacture, it is desirable to develop separate PES curves for each head of a disk drive.
The present invention allows for the development of separate PES curves for each head of a disk drive.
Before continuing, it must be noted that, for the present invention, the term PES signal is defined as the error signal that is used to place the head on or near its proper position (relative to the track centerline) after the head has already been positioned on the proper track (i.e., during track following). In other words, the PES signal as defined herein does not include the coarse positioning information required to position the head on the correct track. Of course, the PES signal as defined herein may be used in combination with the coarse positioning information to properly position the head on the disk.
To properly set forth the background of the invention, a linear disk drive system will now be described and certain conventions will be adopted. Importantly, these conventions will be used with regard to describing the present invention.
A standard disk drive, generally designated 10, is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a head 20 mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 also contains a voice coil motor 28 which moves the head 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and head 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device.
The disk drive 10 typically includes a plurality of disks 12 and, therefore, a plurality of corresponding actuator arm assemblies 18. However, it is also possible for the disk drive 10 to include a single disk 12 as shown in FIG. 1.
Referring now to FIG. 2, data is stored on the disk 12 within a number of concentric radial tracks 40 (or cylinders). Some tracks 40 solely contain servo information while other tracks contain both servo information and data. Each track 40 is divided into a plurality of sectors 42. In tracks 40 which contain both servo information and data, each sector 42 is further divided into a servo region 44 and a data region 46.
The servo regions 44 of the disk 12 are used to, among other things, accurately position head 20 (the head 20 is shown in FIG. 1) so that data can be properly written onto and read from the disk 12. The data regions 46 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
Each track 40 has a centerline 48. To accurately write and read data to and from the data region 46 of the disk 12, it is desirable to maintain the head 20 in a relatively fixed position with respect to a given track's centerline 48 during each of the writing and reading procedures. For simplicity and for purposes of demonstrating the invention, it will be assumed that the head 20 should be positioned on, or substantially on, a given track's centerline 48 to accurately read data from and write data to the data region 46 of that track 40. It should be noted, however, that the invention described herein is equally applicable to those systems which incorporate a read or write offset from the track centerline, as will be understood by those skilled in the art.
To assist in controlling the position of the head 20 relative to the track centerline 48, the servo region 44 contains, among other things, servo information in the form of servo patterns 50 comprised of groups of servo bursts A, B, C, D as shown in FIG. 3. The servo bursts A, B, C, D are accurately positioned relative to the centerline 48 of each track 40, are typically written on the disk 12 during the manufacturing process using a servo track writer ("STW") and, unlike information in the data region 46, may not be over-written or erased during normal operation of the disk drive 10.
As shown in FIG. 3, the A and B burst pairs define what are conventionally known as the Norms. The difference in amplitude between the A and B bursts at a particular head position is defined as the Norms signal (in units of volts) and is represented by N=A-B. A Norms curve can be developed to represent Norms signals for head positions across an entire track.
Likewise, the C and D burst pairs define what are conventionally known as the Quads. The difference in amplitude between the C and D bursts at a particular head position is defined as the Quads signal (in units of volts) and is represented by Q=C-D. A Quads curve can be developed to represent Quads signals for head positions across an entire track.
During the manufacturing process of the disk drive 10, a servo-track writer (not shown) is used to write servo bursts A, B, C, D onto each of the servo regions 44 of the disk 12. As shown in FIG. 3, each track has a track width ("TW") and the distance between each pair of horizontal grid lines represents 1/2 of a track width (or "TW/2"). Accordingly, each of the servo bursts A, B, C, D depicted in FIG. 3 spans a distance equal to one track width. Additionally, as depicted in FIG. 3, the head 20 has a width approximately equal to one track width.
It should be noted that the term track width, as used in connection with describing the present invention, is defined as two STW step widths, as will be understood by those skilled in the art.
With reference to track n, servo bursts A and B are displaced on either side of the centerline 48 of track n. Both servo bursts A and B "contact" the centerline 48 of track n along one oftheir "ends." Similarly, servo bursts C and D are displaced on either side of the intersection of track n-1 and track n, which is a half track away from the centerline 48 of track n. Both servo bursts C and D "contact" the intersection of track n-1 and track n along one of their "ends."
Additional groups of servo bursts A, B, C, D (i.e., the servo bursts which correspond with track n+2 and track n+4) are in radial alignment with the group of servo bursts A, B, C, D described in connection with track n, as shown in FIG. 3. Accordingly, each one of the A servo bursts are radially aligned with one another, and radially adjacent A servo bursts are spaced apart by the distance of one track width. For example, servo burst A of track n+2 is in radial alignment with servo burst A of track n and is spaced therefrom by the distance of one track width.
Similarly, all of the B, C and D bursts are respectively radially aligned with one another. Furthermore, radially adjacent B. , and D bursts are respectively spaced apart by the distance of one track width. For example, servo burst B of track n+2 is in radial alignment with servo burst B of track n and is spaced therefrom by the distance of one track width. Likewise, the radial alignment and spacing of corresponding servo bursts C and D follow suit. It should be noted that the space between the servo bursts is not written upon by the servo-track writer.
With reference to FIGS. 1-3, as the head 20 is positioned over a track 40, it reads the servo information contained in the servo regions 44 of the track 40, one servo region 44 at a time. The servo information is used to, among other things, generate PES signals as a function of the misalignment between the head 20 and the track centerline 48. The PES signals are input through a microprocessor which performs calculations and outputs a servo compensation signal which controls the voice-coil motor 28 to place the head 20 over the track centerline 48.
As will be understood by those skilled in the art, because the system shown in FIGS. 1-3 is a linear system, its PES curve is based upon the position of the head 20 with respect to the A and B bursts. More specifically, in such a system, the PES curve is based upon the Norms curve, while the Quads curve is not used for track following.
Referring again to FIG. 3, when the head 20 is positioned exactly over the centerline 48 of track n, one-half of the A burst will be read followed by one-half of the B burst, and their amplitudes will be equal. In such case, the Norms signal will be zero. It follows that the PES signal, because there is no misalignment of the head relative to the track centerline, will also be zero. As the head 20 moves off of the track centerline, the amplitude of one burst will increase while the amplitude of the other burst will decrease, depending on the direction of misalignment. By knowing the amplitude of the Norms signal at different head positions, a Norms curve may be developed.
The Quads curve can be developed in a similar manner. Specifically, if the head 20 is positioned exactly one-half track above the centerline of track n (at the intersection of track n-1 and track n), one-half of the C burst will be read followed by one-half of the D burst. As the head 20 moves off of the intersection of track n-1 and track n, the amplitude of the either the C burst or the D burst will increase while the other will decrease, depending on the direction in which the head 20 is moved.
FIG. 4 shows ideal Norms and Quads curves for the head 20 described in connection with FIG. 3. In FIG. 4, the x-axis is used to record positional distances along the disk 12 and is divided into units of tracks. The y-axis is given in units of volts. As is clear from FIG. 4, the ideal Norms and Quads curves for the head 20 are linear.
In developing the Norms and Quads curves of FIG. 4 using the servo burst patterns of FIG. 3, certain conventions were adopted. First, it was assumed that the disk 12 is rotating in the direction of arrow 100. Second, the head 20 was considered to be moving towards the outer diameter ("OD") of the disk 12 when it moved up the page and towards the inner diameter ("ID") of the disk 12 when it moved down the page (see the appropriate arrows which are labeled ID and OD in FIG. 3). Finally, a track was defined as an Even track if its A burst was on the OD side of the track centerline (for example, tracks n, n+2, n+4, etc.). Conversely, a track was defined as an Odd track if its A burst was on the ID side of the track centerline (for example, tracks n-1, n+1, n+3, etc.). As mentioned above, these conventions will be used in connection with describing the present invention in the section of this document entitled detailed description of the preferred embodiment.
Upon review of FIG. 4, one sees that when head 20 is properly placed over a track centerline, the resulting Norms signal (N=A-B) is zero. If the ideal head is slightly off the track centerline of an Even track in a direction towards the OD of the disk, the resulting amplitude of the Norms signal is positive. On the other hand, if the ideal head is slightly off the track centerline of an Even track in a direction towards the ID of the disk, the resulting amplitude of the Norms signal is negative. Conversely, if the ideal head is slightly off the track centerline of an Odd track in a direction towards the OD of the disk, the resulting amplitude of the Norms signal is negative. While if the ideal head is off the track centerline of an Odd track in a direction towards the ID of the disk, the resulting amplitude of the Norms signal is positive.
As far as the Quads signal goes (shown in dotted lines in FIG. 4), its amplitude is zero when the ideal head is a distance of +/-1/2 track from each track centerline and the absolute value of its amplitude is a maximum when the head is on the track centerline.
Generally, as one can observe from FIG. 4, the Norms curve and the Quads curve are identical in shape but are 90 degrees out of phase from one another. Hence, as will be understood by those skilled in the art, in a situation where the Norms and Quads curves are entirely linear (as shown in FIG. 4), the PES curve can be based entirely upon the Norms curve. In other words, the Quads curve is not needed for track following.
In the past, thin-film inductive ("TFI") heads were used to perform both the read and write functions of a disk drive. TFI heads, like the head 20 described in connection with FIGS. 1-4 above, are designed to be one track-width wide and, generally, have a linear response. Therefore, as is understood by those skilled in the art, PES curves for TFI heads are substantially linear and are based upon their Norms curve.
Relatively recently, there has been a trend to use magneto-resistive ("MR") heads instead of TFI heads to perform a disk drive's read functions. One of the main reasons for the switch is due to the greater sensitivity of MR heads over TFI heads. As a result, areal densities have dramatically increased.
MR heads have been designed so that their width is much less than one track-width, usually around a half-track width wide. Unfortunately, however, this has caused the Norms signals associated with MR heads to be, in-part, non-linear.
For example, if an MR head having a width of only a half-track was moved from the track centerline of track n:n FIG. 3 upwards slightly more than half a track-width, the head would only be able to read information from the A servo burst, while the information from the B servo-burst would no longer be able to be read. This would cause some non-linearity in the Norms curve and, hence, the PES curve. Therefore, the Norms curve cannot be used, without some manipulation, to develop a PES curve for an MR head having a width of one-half a track.
Additionally, because head widths vary from head-to-head during their manufacture, heads may have Norms curves that look slightly different from one another.
Accordingly, there is a need to develop a relatively linear PES curve which takes the non-linear nature of MR heads into account so that such heads are more accurately positioned over data regions. Also, there is a need to develop a PES curve for each head in a disk drive which contains multiple heads to account for the variations in the widths of the such heads. The present invention, among other things, is designed to meet the aforementioned needs and to overcome the aforementioned problems.